In recent years, the use of frequency hopped nets have been developed to provide for secure and reliable digital communication systems. It has been noted that frequency hopped nets are able to maintain intelligible communication with as much as 20 percent of their channels jammed, and for this reason, it has been possible to operate frequency hopped (FH) net transmissions in very cluttered spectral regions.
Since the frequency hopped rates tend to be generally in a fairly low, sweeping level, about 50-500 Hz, then it was seen that radio stations and other potential interference sources having band widths greater than 500 Hz tend to operate so as to considerably reduce the sensitivity of "fourth law" type detectors. Fourth law detectors are detectors whose output signal-noise ratio is proportional to the 4th power of the input signal-noise ratio.
Due to this limitation, practical frequency hopping (FH) detection schemes have often used channelizers which break up the frequency spectrum into smaller-sized band widths and allow the operator to keep track of narrow band interference sources. These methods relay on the high, instantaneous signal-to-noise ratio in the occupied channel for the detection capability. It should be noted, however, that channelizers suffer from a lack of efficient, automatic detection algorithms. Thus, it was considered useful to develop a hybrid detector which would include both channelizing circuitry and automatic feature detection circuitry which would still retain the advantage of each of these two systems.
Frequency hop radio transmissions create processing gain by utilizing a large number of independent hop frequency locations, for example, the Jaguar radio manufactured by Racal-Tactkom, Ltd. Berkshire, England, makes use of up to 2,000 different hop locations. It is generally seen that the input band width, W, of a frequency hop detector unit is much larger than the width of the binary phase shift keying envelop (BPSK).
Therefore, the BPSK modulation can be collapsed and the noise decorrelated by a delay-and-complex conjugate multiply stage in which the delay is set to approximately 1/W. This method is utilized by the type of hop rate detector known as the MODAC hop rate detector, shown in FIG. 2B. The MODAC was developed by Pacific Sierra Research located at Los Angeles, Calif.
The output signal of the MODAC hop rate detector is seen to be a random, complex, phase-shift keying signal (PSKS) with transitions occurring at the hop rate 1/T.sub.h where T.sub.h=hop dwell time period. In this situation, the PSK signal-to-noise ratio is significantly improved by low pass filtering near the hop rate, and a spectral line (at the hop rate) is generated by another delay-and-complete conjugate multiply stage in which the delay is set to approximately T.sub.h /2.
In the AC hop rate detector, shown in FIG. 2A, and in the AC radiometer, shown in FIG. 3, the input band is divided into two "half bands", and the BPSK modulation is collapsed by "magnitude squaring". The outputs of the squaring devices are then subtracted to form a bipolar signal fed to a difference amplifier. The difference amplifier is AC-coupled (eliminating the DC) to the second stage of the detector because of a direct current (DC), a term generated by the magnitude squaring of the noise involved with the information signal.
The input signal hops randomly between the two half bands, and thus the first stage output signal is a random, "direct sequence" (DS) wave form with transitions occurring at the hop rate. As with the MODAC detector, the direct sequence (DS) signal-to-noise ratio is significantly improved by low-pass filtering (LPF) near the hop rate.
The AC radiometer (FIG. 3) collapses the direct sequence (DS) signal by squaring, and then utilizes an integrator or low-pass filter for detection. The AC hop rate detector (of FIG. 2) generates a spectral line at the hop rate with use of a delay-and-mix circuit.
Up until the first low-pass filter, the AC hop rate detector and the AC radiometer are identical. However, the AC hop rate detector delay-and-mixer (FIG. 2A) generates a square wave with one-half the input signal amplitude, and thus, one-fourth the signal power. The power in the fundamental of the square wave is further reduced by a factor of 4/.pi..sup.2.
It follows that the AC radiometer (FIG. 3) output signal-to-noise ratio is approximately 9 dB greater than that of the AC hop rate detector (FIG. 2A).
Additionally, analysis has been made to indicate that, for low-input signal-to-noise ratio the AC hop rate detector (FIG. 2A) outperforms the MODAC hop rate detector (FIG. 2B) by the amount of 3 dB.
Thus, spectral analysis techniques, it may be understood, will not always reveal the "presence" of hybrid FH/DS (frequency hopping/direct sequence) signals because of the inherent covert nature of these signals. However, the class of fourth law detectors (such as the MODAC) has been shown to be useful when used with all types of frequency hopped signals.
For example, the simplified AC radiometer (FIG. 3) generates a DC level when frequency hopped (FH) signals are "present", thus reducing the signal present/signal absent decision to a comparison with a set threshold. In addition to "signal presence" the "hop rate" can be determined with both the AC hop rate detector (FIG. 2A) and the MODAC hop rate detector (FIG. 2B). Each of these detectors generates a spectral line at the hop rate, which can be detected and characterized by ordinary spectral analysis techniques.
Another class of detectors which has been shown to be useful against frequency hopped signals are those which utilized "channeling" techniques. At any given point in time, the hybrid FH/DS signal is present in one channel only, thus providing a much higher instantaneous signal-to-noise ratio which can be exploited by various methods.
Radiometers have been utilized extensively for the purpose of detecting various spread-spectrum signal types, but generally suffer in the presence of narrow-band interference sources. Further channelizers have been utilized where narrow-band interference signal operations is a problem, as it normally is with frequency hopped nets. However, the channelizers suffer from a lack of efficient detection algorithms. Thus, in cluttered frequency bands, in which frequency hopped communication networks operate, it is essentially desireable to have a detector which has considerable immunity to narrow-band (NB) signal interference.
Thus, it is an object of the present invention to provide "a signal presence detector" for modulated or unmodulated frequency hopped (FH) signals, and which helps to provide considerable immunity to narrow-band interference sources.